The phenomenon of magnetostriction has been widely employed in linear distance and position measuring devices. A magnet located near or around a magnetostrictive wire marks the location to be measured. Such devices can operate with either mechanical or electrical excitation. When an acoustical/mechanical strain propagating along the wire reaches the area of influence of the magnet, an electrical signal is generated in the wire. Conversely, when an electrical signal propagating along the wire reaches the area of influence of the magnet, a torsional strain is generated in the wire.
Such linear position detectors using a magnet mounted in a float have been utilized as liquid level detectors to provide an indication of a level of liquid within a tank, such as an underground tank. The position of the magnet, and hence, the liquid level, is determined as a function of time required for a torsional disturbance to propagate from one end of the wire through the area of influence of the magnet, in the case of mechanical excitation, or from the position of the magnet to a sensing apparatus located at one end of the wire in the case of electrical excitation.
Other types of magnetostrictive position measuring devices utilize a reflective termination at the foot end of the magnetostrictive wire. Such devices measure the difference between the propagation times of a pulse from the magnet position to the foot of the wire and reflected back to the head of the device and a pulse traveling on the wire directly from the magnet to the head.
As shown in U.S. Pat. Nos. 4,839,590; 5,017,867; 5,050,430; and 5,253,521, all of which are assigned to the assignee of the present invention, such magnetostrictive devices typically include an elongated, small diameter tube, typically made of stainless steel, on which a movable magnet is mounted for providing an indication of a liquid level. A head and cap are mounted on one end of the tube, typically above the liquid level, and enclose electronic components, such as terminal connections and a signal conditioning circuit used to supply or output signals to and from the magnetostrictive wire in the tube.
At least one transducer is mounted at one end of the magnetostrictive wire, usually within a head or housing mounted at one end of the tube, to generate the torsional pulse for propagation along the wire or to detect a torsional pulse generated through the interaction of an initial excitation signal propagated along the wire and the movable magnet. However, such transducers require special mounts, substrates and/or clamps for attachment to the magnetostrictive wire or to dispose the transducer in a surrounding relationship with respect to the wire. These additional components add to the overall cost of the position measurement device as well as increasing the assembly time of the device. More importantly, such components consume a considerable amount of space thereby requiring the typically enlarged diameter head mounted at one end of the tube surrounding the magnetostrictive wire. This enlarged head limits the application of a magnetostrictive position measurement apparatus only to those applications which can accommodate the large diameter head. Other applications, such as underground tanks having small diameter access ports, will not accommodate the enlarged head without major modification.
Thus, it would be desirable to provide a magnetostrictive position measurement apparatus which is capable of detecting a position of a movable object without a transducer. It would also be desirable to provide a magnetostrictive position measurement apparatus which is capable of determining the position of a movable magnet associated with a movable component, such as liquid level within a tank, without using a transducer while still having sufficient sensitivity to detect signals propagating along a magnetostrictive wire. It would be desirable to provide a magnetostrictive position measurement apparatus which has a small overall size or diameter for use in many different applications.
It would be desirable to provide a magnetostrictive position measurement apparatus which eliminates the possibility of failure of the transducer or the effects of any transducer based temperature drift.